BEIJING, Jan. 11 (Xinhua) -- A Foreign Ministry spokesman said Friday that Chinese military planes were on "routine flights" in relevant airspace over the East China Sea.

Spokesman Hong Lei made the remarks at a press briefing in response to media reports that Japan sent fighter jets to head off a number of Chinese military planes spotted in Japan's "air defense identification zone" over the East China Sea on Thursday.

"China firmly opposes Japan's moves to gratuitously escalate the situation and create tensions," Hong said.

Relations between Japan and China have soured since the Japanese government said it would "purchase" part of the Diaoyu Islands in the East China Sea in 2012. China insists that the islands are part of its inherent territory.